1. Field of the Invention
The present invention relates to an improved grinding method and apparatus for grinding a workpiece, and more particularly, relates to an improved grinding method and apparatus with a function for compensating a detected position of a wheel head for a thermal displacement.
2. Discussion of the Prior Art
Heretofore, in a conventional cylindrical grinding machine, a wheel head supporting a rotating grinding wheel 19 is advanced toward a workpiece W, so that cylindrical sections Wa, Wb and Wc of the workpiece W which is supported between a center 15a of a work head and a center 16a of a tailstock, are ground with the grinding wheel 19, as shown in FIG. 1. As shown in FIG. 2, the wheel head is infed as reducing its infeed rate in order such as at a rough grinding infeed rate G1, a fine grinding infeed rate G2 and a finish grinding infeed rate G3. Therefore, the workpiece W is ground with the grinding wheel 19, wherein the position of the wheel head is represented by the line G. Further, to compare the position of the wheel head with the diameter of the workpiece W, the diameter of the workpiece W is converted into a value S corresponding thereto. Namely, the value S represented by the line H is calculated by the following equation: EQU S=(R+B)/2
where, B represents the diameter of the workpiece W directly measured by an in-process measuring device (not shown), and R represents the diameter of the grinding wheel 19 which has been beforehand measured and stored in a numerical controller. However, it is general that the line G does not coincide with the line H at the end portions of the lines G and H. This is because there takes place a thermal displacement between the workpiece W and the wheel head with changes in the temperature of machine components.
For this reason, in a conventional grinding machine using the in-process measuring device for measuring the diameter of the workpiece W during the grinding operation, the difference d between the lines G and H indicated in FIG. 2 at the ending time of the finish grinding operation is calculated based on the diameter B of the workpiece W measured by the measuring device and the absolute position S of the wheel head detected by a suitable position detector. And, position data for the wheel head detected by the suitable position detector is compensated for the calculated difference d, whereby the error of the detected position data for the wheel head primarily due to the thermal displacement can be compensated for. In the successive grinding operations on the sections Wb and Wc of the workpiece W, the indirect-sizing grinding operation which does not use the in-process measuring device is carried out using the compensated position data for the wheel head. Therefore, in the successive grinding operations, the machining error caused by the thermal displacement can be diminished, and because of the indirect-sizing grinding operation, the machining time can be shortened.
However, the above-aforementioned difference d is influenced not only by the thermal displacement but also by the bending or flexing of the workpiece W due to the grinding resistance. Because the position of the wheel head involving the thermal displacement and the diameter of the workpiece W are detected or measured while the wheel head is being infed against the workpiece W and while the workpiece W is being bent by the grinding resistance, the diameter of the workpiece W cannot be measured accurately.